The present invention relates to combustion turbines as may be employed in a variety of uses, such as industrial processes, electric power generation, or aircraft engines. More particularly, the present invention is directed to combustors employed in combustion turbines for heating motive gases which drive the turbine.
In general terms, a typical prior art combustion turbine comprises three sections: a compressor section, a combustor section, and a turbine section. Air drawn into the compressor section is compressed, increasing its temperature and density. The compressed air from the compressor section flows through the combustor section where the temperature of the air mass is further increased. From the combustor section the hot pressurized gases flow into the turbine section where the energy of the expanding gases is transformed into rotational motion of a turbine rotor.
A typical combustor section comprises a plurality of combustors arranged in an annular array about the circumference of the combustion turbine. In conventional combustor technology, pressurized gases flowing from the compressor section are heated by a diffusion flame in the combustor before passing to the turbine section. In the diffusion flame technique, fuel is sprayed into the upstream end of a combustor by means of a nozzle. The flame is maintained immediately downstream of the nozzle by strong aerodynamic recirculation. The lack of thorough mixing of the fuel results in pockets of high fuel concentration and correspondingly high combustion reaction temperatures. Because the reaction temperature is high, hot gases flowing from the combustion reaction must be diluted downstream by cool air so as to prevent damage to turbine components positioned downstream. In addition, the flame diffusion technique produces emissions with significant levels of undesirable chemical compounds, including NO.sub.x.
NO.sub.x results from two basic mechanisms. Thermal No.sub.x is produced from the combination of nitrogen and oxygen in the fuel oxidizer (air) during and after the combustion process when the temperature level is sufficiently high to permit the overall reaction of EQU N.sub.2 +O.sub.2 .fwdarw.2NO
to occur at a measurable rate. The thermal NO.sub.x reaction occurs for all combustion processes using air and is essentially independent of the fuel.
NO.sub.x is also formed from fuel-bound nitrogen, which forms NO-type compounds in the combustion process in a manner somewhat analogous to the formation of CO and CO.sub.2 from fuel carbon and H.sub.2 O from fuel hydrogen. The differences between the two mechanisms for forming NO.sub.x lie in the time and temperature of the combustion process. Fuel-bound nitrogen compounds appear virtually simultaneously with the CO, CO.sub.2, and H.sub.2 O, while the formation of NO.sub.x from the oxidizer appears later and is governed by a kinetic rate mechanism.
Increasing environmental awareness has resulted in more stringent emission standards for NO.sub.x. The more stringent standards are leading to development of improved combustor technologies. One such improvement is a premixing, pre-vaporizing combustor. In this type of combustor, fuel is sprayed into a fuel preparation zone where it is thoroughly mixed to achieve a homogeneous concentration which is everywhere within definite limits of the mean concentration. Additionally, a certain amount of fuel is vaporized in the fuel preparation zone. Fuel combustion occurs at a point downstream from the fuel preparation zone. The substantially uniform fuel concentration achieved in the fuel preparation zone results in a uniform reaction temperature which may be limited to approximately 2000.degree. to 3000.degree. F. Due to the uniformity of the combustion, the pre-mixing, pre-vaporizing combustor produces lower levels of thermal NO.sub.x than does a conventional combustor using the same amount of fuel. NO.sub.x formed from fuel-bound nitrogen is tolerable because of the comparatively low nitrogen content of the typical petroleum fuel utilized.
The increased environmental awareness of recent years regarding emissions standards has been accompanied by a recognition of the limited availability of petroleum fuels. Consequently, a trend has developed focusing on the use of nonpetroleum fuels for combustion turbines. Nonpetroleum fuels typically have a higher nitrogen content than do petroleum fuels. For example, a typical petroleum fuel might have a nitrogen content of 0.1% by weight, while coal-derived liquids contain fuel-bound nitrogen up to 1% by weight and oil shale-derived liquid fuels contain fuel-bound nitrogen up to 2% by weight. Because they do not inhibit NO.sub.x formed from fuel-bound nitrogen, premixing, pre-vaporizing combustors would likely fail the stringent NO.sub.x standards when operated with nonpetroleum fuels.
Hence, it appears that known prior art combustors do not adequately provide for low-NO.sub.x emissions when operated with nonpetroleum fuels.